1. Technical Field
The present invention relates to the field of synchronization of time base units. In particular, the present invention relates to a method of synchronizing a first and a second time base unit in a receiving station, to a communication system for synchronizing a first and a second time base unit, to a receiving station for a communication system for synchronizing a first and a second time base unit and to a software program product for performing a synchronization of a first and a second time base unit.
2. Description of the Related Art
Due to emerging cellular standards such as Universal Mobile Telecommunications Systems (UMTS), General System for Mobile Communications (GSM), Wireless Local Area Network (WLAN), Time Division Synchronous Code Division Multiple Access (TD-SCDMA) and Bluetooth there is more and more need for so-called multi-mode mobile phones, i.e. phones which support not only one air interface, but at least two. In many cases, these phones have to support inter-system operations, i.e. they have to be capable of making voice or data calls on all supported standards. Sometimes, even multi-standard handovers, i.e. a transition between the systems during a call, have to be supported, for example, between UMTS and GSM.
To facilitate these handovers, the maintenance of two or more time bases is needed to be able to synchronize with the different systems. A high accuracy crystal clock source, such as a voltage controlled temperature compensated crystal oscillator (VCTCXO), is generally used in the receiving station or multi-mode mobile phone in order to maintain a time base for every system the receiving station supports.
In some modes of operation, such as a paging mode, when the receiving station is waiting for incoming calls, the receiving station may not have to perform any tasks for periods of up to several seconds. However, due to power saving considerations, today's mobile phones or receiving stations use refined power down modes as long as no direct connection with the network exists. When the receiving station enters a power saving mode or sleep mode after not having received an incoming call or other event for a certain period of time, the receiving station must still be able to receive appropriate time slot and other service information during the idle time. In other words, the various time bases in the receiving station must still be maintained, even while in these stand-by or sleeping modes.
If a receiving station is in such a so-called sleep mode, it usually switches off most internal components and initializes wake-up counters, which trigger the re-start of these components.
For integrated circuits used in multi-mode mobile phones, such as complimentary metal oxide semiconductor integrated circuits (CMOS ICs), power consumption is directly related to clock frequency. Therefore, high frequency crystal clock sources and the circuits connected to these clock sources dissipate a significant amount of power. In order to reduce the power consumption, the internal time bases are maintained by a low accuracy oscillator, such as a crystal oscillator, with a comparatively low frequency, typically 32 KHz, during the sleep mode. After wake-up from sleep mode, all internal time bases need to be readjusted accurately, which requires a complex interaction of several software and hardware components in case of a multi-mode phone.